Move STT and image generation to the assistant UI

Fix image generation result page for non-authenticated users
Add an STT task type in the assistant UI and implement a specific input form
Add an image generation task type in the assistant UI and implement a specific output field
Support STT and image generation in the assistant task result page
Use the assistant result page as target for STT and image generation notifications
Allow to schedule (instead of runOrSchedule) image generation tasks (when canceling a sync task with the "run in the background" button)
Harmonize image generation workflow with the rest: don't show the result widget if it has been scheduled, set notifyReady to true automatically
Use @nextcloud/moment instead of humanize-duration for time estimation calculations and human readable display
Downgrade @nextcloud/moment to avoid compilation warning
Set max height of NcRichContenteditable to 300px, fix top/bottom borders not visible in some cases
Set background color of modal close button to make it more visible when it overlaps with content while scrolling

Open questions:

In the image gen and STT pickers, should we just use the openAssistantForm function?

nextcloud / assistant